1. Field of the Invention
This invention relates to a process for preparing .alpha.,.beta.-unsaturated aldehydes of the general formula (I) ##STR6## in which R.sup.1, R.sup.2 and R.sup.3 are independently a hydrogen atom, or an alkyl or alkenyl group with or without being substituted with a lower acyloxy group. The .alpha.,.beta.-unsaturated aldehydes of the general formula (I) prepared according to the process of the invention are useful as intermediates for preparing vitamin A used as a drug or a feed additive, or as fragrance chemicals.
2. Prior Art
.alpha.,.beta.-Unsaturated aldehydes are known as prepared according to the following processes.
(1) A process of preparing 4-acetoxy-2-methyl-2-butenal which comprises reacting 4-chloro-3-methyl-2-butenyl acetate with dimethyl sulfoxide in the presence of dipotassium hydrogen phosphate, potassium dihydrogen phosphate and sodium bromide (The Journal of Organic Chemistry, Vol. 44, pp. 1716-1717 (1979)).
(2) A process of preparing 2,6-dimethyl-2,6-heptadienal in which 7-chloro-2,6-dimethyl-1,5-heptadiene is reacted with dimethylsulfoxide in the presence of silver tetrafluoroborate, and then triethylamine is added to the resulting reaction mixture (Tetrahedron Letters, pp. 917-920 (1974)).
(3) A process of preparing 3,7-dimethyl-2,6-octadienal by reaction between 1-chloro-3,7-dimethyl-2,6-octadiene and potassium 2-propanenitronate (British Pat. No. 1,291,246).
(4) A process of preparing 8-acetoxy-2,6-dimethyl-2,6-octadienal which comprises oxidizing 3,7-dimethyl-2,6-octadienyl acetate with selenium dioxide (Tetrahedron Letters, pp. 281-284 (1973)).
These prior art processes have problems from the standpoint of the industrial production of .alpha.,.beta.-unsaturated aldehydes. More particularly, the process (1) requires the presence of a brominating agent and a buffering agent in the reaction system, thus requiring a plurality of reagents. Silver tetrafluoroborate used in large amounts on the process (2) is expensive. Potassium 2-propanenitronate used as an oxidizing agent in the process (3) is explosive and thus, care should be taken to its handling. Selenium dioxide used in the process (4) is toxic and sublimable, so that it has to be handled with great care. Accordingly, it is difficult to carry out the process (4) on an industrial scale.